1. Field of the Invention
The present invention relates to satellite angular momentum control systems having at least one magnet-superconductor momentum storage device.
More particularly, the present invention relates to satellite angular momentum control systems for maintaining a satellite in a given attitude and spin orientation, for changing a satellite attitude and/or spin orientation, for measuring a satellite angular velocity vector and for dynamic bias for orbital yaw steering, where the momentum storage or gyro system incorporates at least one, and preferably a plurality, magnet-superconductor momentum storage or flywheel devices. Moreover, the present invention relates to an attitude control system.
2. Description of the Related Art
Satellites are aligned in orbit by to general methods. One method involves using attitude and spin jets to orient or change the orientation of a satellite in orbit. The second method, and the method to which this invention pertains, involves the use of momentum storage devices the balance of which maintains the satellite in a given orientation with respect to the sun or the earth or any other fixed object. The amount of momentum stored in each device controls the exact orientation of the satellite as well as its spin axis. To change the orientation and spin of the satellite, the momentum of one or all of the momentum storage devices are changed which in turn changes the angular momentum of the satellite resulting in its orientation change.
Flywheels have been well-known as one of the oldest ancient mechanical designs in human history. Historically, the first flywheel dates back to 3000 BC, when the flywheel was recognized as the "potter's wheel". Essentially being a mechanical battery, flywheel energy storage (FES) system, many believe, could be one of the most efficient means to solve two critical problems faced by modern society: the rapid increase in the use of energy and the consequent impact of energy consumption on the environment.
Of particular concern to a flywheel energy storage device is its overall efficiency, which is dictated by four major factors: (1) motor/generator conversion efficiency; (2) power conditioning system efficiency; (3) windage drag; and (4) flywheel bearing efficiency.
Recent developments in new materials and magnetic bearings using electromagnetic levitation resurrect the interests of scientists and engineers in advancing the flywheel technology for energy storage applications.
Conventional mechanical bearings used in conjunction with high rotational speed devices are subject to metal wear, noise, vibration and friction heating problems. These problems can often lead to seizure or other failure of the bearing. In addition, mechanical bearings often require lubricants which fail in severe environments such as those commonly encountered in outer space. Failure of conventional liquid lubricants in outer space is usually due to the vacuum conditions that cause the lubricants to out gas, leaving bearing surfaces dry and resulting in the ultimate failure of the bearings. Additionally, in outer space, temperatures are very low so most lubricants solidify and simply do not function as lubricants.
As a result of these and other shortcomings, there has been considerable emphasis on the developments of alternatives to mechanical bearings. For example, work has been done to develop more efficient air bearings, as well as magnetically suspended bearings.
One problem with air bearings is that they require a complete pneumatic system, including pumps, valves, seals, and conduits, for their operation. Another shortcoming of air bearings is that they result in a continuous energy loss. For example, a high speed cryo-cooler system in outer space applications, would suffer a 10-20 watt energy loss due to bearing friction losses. Even in non-space applications, use of an air system adds significant cost, size, and weight to the bearing package and introduces various reliability problems normally associated with pneumatic system components.
Because of the fine tolerances required, which are on the order of one ten-thousandth of an inch, air bearings themselves are difficult to manufacture, and thus expensive. Furthermore, air bearings are highly vulnerable to contaminants. A particle of dust as small as four ten-thousandths of an inch can interfere with air gaps and clog pores of graphite or other diffusive coatings.
One obvious approach toward reducing friction losses from two relatively moving surfaces is to exert supporting force without physical contacts. Thus, magnetically suspended bearings have been developed as an alternative to air bearings.
While active magnetic bearing systems are now a well-developed and widely accepted technology, no intrinsic stability exists for a magnetized body under the influence of an external magnetic field. This phenomenon is known as Earnshaw's theorem and occurs in any potential satisfying the Laplace equation, which includes electric, magnetic, and gravitational potentials.
Thus, in an active magnetic bearing, feedback control is applied to an electromagnet or combinations of permanent magnets and electromagnets to accomplish dynamic stability.
Because permanent magnet magnetic field strength is generally limited, the use of permanent magnets is limited to applications where very small forces are adequate. Electromagnets can supply considerably more magnetic force than comparable permanent magnets, and their magnetic fields can be altered by use of feedback control systems, and are thus generally preferred.
Unfortunately, the required feedback control systems with the attendant complex sensor and actuator electronics, add considerably to the cost, size, and operational complexity of an active magnetic bearing system.
It has been appreciated for years that magnetic fields strongly interact with superconducting materials. For example, stable levitation of a superconductor in an external magnetic field has long been demonstrated. Levitation of low temperature superconductors (LTS) by a charged superconducting coil and permanent magnet levitated above a bowl-shaped LTS are some classic demonstrations of superconductivity. Such systems are technically considered passive bearings, since there is no explicit use of feedback control. However, such systems are effectively active, that is active per se, since the intrinsic opposite magnetization constantly induced is fulfilling the same function of maintaining a dynamically stable levitation as a feedback control system.
The most distinctive property of a superconductive material is its loss of electrical resistance when it is at or below a critical temperature. This critical temperature is an intrinsic property of the material and is referred to as the superconducting transition temperature of the material, T.sub.c.
Recent research activities have brought the discovery of "high temperature superconducting" ("HTS") compounds. HTS compounds are those which superconduct at and below a critical temperature, T.sub.c, which is above the boiling point temperature of nitrogen.
Following the discovery of superconductivity in a rare earth-alkaline earth-Cu oxide system of a perovskite crystalline structure, a new class of rare earth-alkaline earth-copper oxides was discovered which are superconductive at temperatures above the boiling point of liquid nitrogen, 77.degree. K. These new rare earth-alkaline earth-copper oxides are now commonly referred to as "123" high-temperature superconductors in reference to the stoichiometry in which the rare earth, alkaline earth, and copper metal atoms are present, namely a ratio of 1:2:3.
Since they are superconductive at temperatures greater than 77.degree. K, the new CuO high temperature superconductors may be cooled with liquid nitrogen, which is a far less costly refrigerant than helium. As a result, the rather complex thermal insulation and helium-recycling systems, necessary to avoid wasting the expensive helium coolant required for the low temperature superconducting material previously known, are no longer necessary. The HTS compounds simplify and enhance the reliability of commercial applications of superconductors. Liquid nitrogen is about 2000 times more efficient to use in terms of cost, when both the refrigerant itself and the associated refrigerant unit design are considered.
Magnetic fields are disclosed for bearings in U.S. Pat. No. 3,810,683. Use of superconductors for support bearings are taught in U.S. Pat. No. 3,378,315, wherein superconducting material is used for a spindle bearing with either permanent magnets or electromagnets providing the supporting magnetic field. U.S. Pat. No. 3,026,151 shows superconducting bearings with the actuator coils likewise formed of superconducting materials.
The recent advances in superconducting materials and the parallel advancements in the field of permanent magnets have made it possible to economically and efficiently couple a superconducting member with a magnetic member to produce highly efficient and relatively inexpensive bearings.
Superconductive materials are of two basic types, designated as Type I and Type II. Efforts have been made in the past to improve magnetic bearing technology by maintaining either the bearing member or the rotating member, or both, in a Type I superconducting state to achieve sufficient magnetic pressure to provide the desired degree of levitation. Unlike Type II superconductors, Type I superconductors are incapable of effecting suspension.
Type I superconductors feature perfect diamagnetism up to a critical applied field, at which point superconductivity is lost and the magnetization of the sample vanishes abruptly. Examples of superconducting bearings of Type I materials can be found in U.S. Pat. Nos. 3,493,274 and 3,026,151. In order to achieve stability in these systems, the bearing structures must rely on either a mechanical rotary support, or must employ superconductors shaped to provide a laterally stable configuration.
The recent discoveries of high temperature superconductors involve Type II materials. Whereas a Type I superconductor completely blocks out magnetic flux from its interior, a phenomenon known as diamagnetism, Type II superconductors allow a certain amount of magnetic flux to penetrate into the interior of the material, producing a suspension effect in addition to a levitation effect. Under such conditions, circulating superconducting currents are established within the superconductor.
A typical example of a system featuring a combination of Type II superconductors and permanent magnets is disclosed in U.S. Pat. No. 4,886,778, which discloses a rotating shaft having two ends, each of which contains a permanent magnet and rotates in a socket clad with superconducting material. The shaft is made to levitate above the sockets by the repulsive forces which exist between the magnets and the superconductors. The incorporation of superconductors into the bearing design offers the possibility of rendering the bearings entirely passive. The design disclosed in U.S. Pat. No. 4,886,778 has the potential for achieving very high rotational speeds, in excess of ten thousand rpm. The interaction between the rotating magnetic axial element and its stationary superconducting support takes place across a gap permeated by a strong magnetic field emanating from permanent magnets embedded in the rotating element.
However, it is desirable to increase the amount of thrust between the superconductor and the magnets with a corresponding increase in the stability. An increased amount of thrust could easily be obtained by utilizing repulsing pairs of magnets in addition. Increased thrust is obtained by increasing the magnetic field on one or both of the magnets, either by utilizing stronger permanent magnets, or increasing the current to an electromagnet. However, as the thrust is increased between repulsing pairs of magnets, the instability of those magnets in the plane normal to the magnet-magnet repulsion axis increases.
U.S. Pat. No. 4,879,537 discloses the use of a superconductor located in the magnet-magnet attraction axis between two attracting magnets. For attracting magnets the instability is along the magnet-magnet attraction axis. However, this system suffers because it is not always desirable to stabilize two attracting magnets by placing a superconductor in the magnet-magnet attraction axis between magnets.
U.S. Pat. No. 5,159,219 discloses the use of high temperature superconductors in close proximity with permanent magnets to make essentially frictionless bearings. Two short comings of the disclosed arrangement are low thrust and low stiffness.
U.S. Pat. No. 5,177,387 discloses the use of an additional magnet to supplement the thrust in a magnet/superconductor system. Since this additional magnet provides the necessary thrust, the high temperature superconductor is left to provide stability. Unfortunately, this arrangement introduces instability from the magnets.
In copending U.S. patent application Ser. No. 08/895,387 filed Jul. 16, 1997; a magnet-superconductor flywheel and levitation systems is disclosed where land-based energy storage flywheels are replaced with magnet-superconductor flywheels having improved properties including stability, storage capacities and storage efficiencies.
The following patents relate to satellite momentum bias control systems using momentum storage devices based on traditional electromechanical devices: U.S. Pat. Nos. 5,826,829, 5,820,079, 5,820,078, 5,814,959, 5,787,368, 5,758,846, 5,752,675, 5,738,309, 5,723,923, 5,692,763, 5,692,707, 5,667,171, 5,608,634, and 5,605,139, incorporated herein by reference.
However, the prior art does not disclose a practical design for creating satellite momentum control systems having lower weight, more stable and longer lasting control systems for satellite attitude and spin control, especially, for low orbit communication satellites utilizing HTS materials. These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its claims and drawings.